TRANSACTIONS OF SECTION C. 561 
below, although in one case the ore has been deposited along the margins of an 
intrusive rock, and in the other case at the surface or in the regions near the 
surface. 
2. Exhibition of Specimens of Tertiary Plutonic Rocks (including Gneisses) 
Jrom the Isle of Rum. By Atrrep Harker, I.A., F.B.S. 
(By permission of the Director of the Geological Survey.) 
Of the plutonic rocks of Tertiary age, which make up about one half of Rum, 
the ultrabasic group is the most important. It includes various peridotites, some 
essentially of olivine, but others containing pyroxenes, and especially anorthite. A 
noteworthy amount of lime and alumina, giving rise to anorthite, is indeed a special 
characteristic of the group. Equally striking is a tendency to separation of the 
more peridotic and the more felspathic portions of the magma, usually with a 
stratiform disposition. With bands of true peridotite alternate others of allivalite, 
a rock consisting of anorthite (predominant) and olivine, and even containing 
seams of pure anorthite rock. Another peculiar type, styled harrisite, is composed 
essentially of olivine (predominant) and anorthite, the olivine occurring here as 
large lustrous black crystals, with good cleavage. 
Later than all these rocks, and intruded beneath them, comes the ewcrite 
group, which shows less variety. The rocks are usually somewhat rich in olivine; 
much of the pyroxene is hypersthene, and the felspar is near anorthite. Still 
later comes the granite group, mostly hornblendic and often with granophyric 
structures. The acid magma has entered into peculiarly intimate relations with 
the eucrite, not only metamorphosing and impregnating that rock, but enclosing 
and partially incorporating portions of it, large and small. The enclosed portions, 
in a half-digested state, have been streaked out by movement, and there has arisen 
a group of well-banded gneisses, closely resembling the Lewisian of the North- 
western Highlands. These Tertiary gneisses are all of the nature of hybrid and 
composite rocks, of which the contributing elements are the eucrite and the 
granite, and their genesis can be traced step by step in the field. 
3. The Lava-Domes of the Eifel. 
By Eywarp GREENLY. 
Associated with those cones and craters of the Hifel which are so remarkably 
preserved, and have suffered so little from denudation, are many bosses and domes 
of massive igneous rock, particularly of phonolite. If these are the denuded necks 
or stumps of volcanoes, they must be far older than the cones and craters. It is 
suggested, however, that they may be really contemporaneous, and have originated 
in the same way as the recent lava-pyramid of Mt. Pelée. This suggestion is 
supported by considerations connected with the distribution of the Trass; and by 
comparison with some other domical and pyramidal masses, 
4. Report on Geological Photographs.—See Reports, p. 242. 
5. Coneretions as the Result of Crystallisation. 
By Professor H. A. Mimrs, £25. 
In the gold districts of the Urals decomposed crystals of iron pyrites are not 
uncommon, which when sawn in half are found to contain a nucleus of gold; fresh 
crystals from the same localities are auriferous, but have the gold uniformly distri- 
buted. Similar nuclei are found in decomposed aikinite (a sulpho-bismuthite of 
lead and copper) at Beresoysk, but here the nucleus of gold is, like the crystal of 
aikinite, rod-shaped. In both cases the metal has become concentrated during the 
“Soar rm of the mineral in which it was contained, 
. Qa0 
